This invention relates to improved threaded connections which are especially designed to divide the total axial load forces of a made-up connection as uniformly as possible among the various different turns of the engaged threads.
In a standard threaded connection between an externally threaded screw and an internally threaded nut, there is an inherent tendency when the screw and nut are tightened against a workpiece for the axial load forces between the screw and nut to be transmitted primarily through the thread turns hwich are located near the axially inner end of the nut, that is, the end adjacent the work. The last turn of the nut at that end takes much more than its fair share of the load, the next successive turn in a direction away from the work takes a smaller amount but still more than the third turn, with this condition continuing to the axially outer end turn which may take very little load. In a highly prestressed connection in which the screw and nut are intentionally tightened to a very high torque maintaining the screw permanently under a heavy tensional force, the non-uniform distribution of load forces between different turns of the engaged threads is exaggerated drastically, and may lead to early failure of the connection at the location of the overloaded axially inner turns.
An arrangement for transferring more of the load in a threaded connection toward the axially outer end of the nut has been disclosed in U.S. patent application Ser. No. 380,439 filed May 20, 1982 by Mark Hattan on "Method Of Forming A fatigue Resistant Threaded Connection", and in U.S. Pat. No. 4,364,136 issued Dec. 21, 1982 to Mark Hattan on "Formation Of Threaded Elements Having A Self-Locking Action". Those prior disclosures show a type of fatigue resistant nut which may consist of two separately formed nut bodies welded or otherwise secured rigidly together and having two similar internal threads aligned axially with one another but turned about their common axis relative to one another so that neither thread is a true helical continuation of the other. The two threads of the nut then engage a standard thread of a coacting screw in a relation giving the connection an interfering self locking action, and also causing axial load forces upon tightening of the nut against a workpiece to initially be taken primarily by the axially outer one of the nut bodies and then be gradually transferred in part to the axially inner nut body as the torque is progressively increased.